This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which automatically perform strain rate analysis within an ultrasonic image.
Many ultrasonic diagnostic procedures in which bodily functions and structures are quantified rely upon clear delineation and definition of the body structures and organs which are being measured. When the quantification or measurement procedure uses static images or a small set of measurements, the delineation of the bodily structure being measured can be done manually. An example of such a procedure is the obstetrical measurements of a developing fetus. Static images of the developing fetus can be acquired during periods when fetal activity is low. Once an image is acquired, only a few circumference or length measurements are usually required to compute development characteristics such as gestational age and anticipated delivery date. These measurements can readily be made manually on the fetal images. Other diagnostic procedures, particularly those involving measurements of the heart and its functioning, present a further set of difficulties. The heart is always beating and hence is always in motion. As it moves, the contours of the heart constantly move and change as the organ contracts and expands. To fully assess many characteristics of cardiac function it is necessary to evaluate many and at times all of the images acquired during the heart cycle (one heartbeat), which can amount to thirty to one hundred and fifty or more images. The structure of interest such as the endocardium, epicardium or valves must then be delineated in each of these images, a painstaking, time-consuming task. Since these structures are constantly in motion, they appear slightly different in each image acquired during the cardiac cycle, and can also vary significantly from one patient to another. While applications such as obstetrical procedures would benefit from a processor which automatically delineates specific anatomy in an ultrasonic image, cardiac diagnosis would benefit even more so.
An ultrasonic cardiac diagnostic technique which has been in development for several years is a parametric imaging method known as strain rate analysis. In strain rate analysis the velocities at consecutive points along an ultrasonic beam line are differentiated to produce a measure of velocity change at points in the image. However, parameters of strain computed in this manner can be arbitrary, as the correspondence of the beam line directions and the anatomy in the image are essentially unrelated, being determined in many instances by the position in which the clinician desires to hold and orient the scanhead. Accordingly it is desirable to be able to compute strain rate parameters in a way that is related to the characteristics of the anatomy under diagnosis.
In accordance with the principles of the present invention, strain rate analysis is performed for ultrasonic images in which the spatial gradient of velocity is calculated in the direction of tissue motion. Strain rate is calculated for cardiac ultrasound images in the direction of motion which, for myocardial images, may be either in the plane of the myocardium or across the myocardium. Strain rate information is calculated for a sequence of images of a heart cycle and displayed for an automatically drawn border such as the endocardial border over the full heart cycle.